you’ll be fine at UM, then.
It’s a shock for American students when they get to college, because in HS there are a LOT of supports: they do homework and it counts in their semester grade, they have short tests, quizzes, exams, some of which they can retake for a higher grade, they may have long research papers or take-home exams, they can get “extra credit” (ie., extra work to get more points), teachers stay in their classroom to help students… It’s not easy, it’s a lot of work, but if you work assiduously and use everything at your disposal, there’s no reason for you to get a C or D. Everything is set up to get you to success - not to get you to fail and see if you survive, the way you might have experienced Upper Secondary School.
Then they get to college and they have to go to office hours to get help, they have to go to tutoring services, so they have to show initiative and resourcefulness (most do, but not all); there’s no extra credit, homework doesn’t necessarily count (depending on subjects and what is calleed “homework”); the professors come and teach then leave - they don’t hang out in the classroom to help, they expect students to be prepared, no goofing around. There’s no “weight” (where you get +.5 to your grade as counted in the GPA because the class is harder than basic college prep classes, or +1 if it’s advanced). Your grade is your grade, so straight A’s are very very rare. So, it can be a shock.
The material is hard and there’s a lot of work to do. It has to be mastered - you need 95% correct for an A so you can’t afford to guess or be halfway there. But if you did well in secondary school, if you did well at the Russian top university, then you should know how to study, how to discipline yourself, what it means to “master” the material. You’ll have access to academic support services and office hours in case a point seems a bit fragile – you should have enough awareness to know you need help and enough strength to go ask for it.
You’ll be okay, even when faced with more competitive students.
NOW, you need to study biology, chemistry, and math. Go to Khan Academy and go back to basics. If you move through the basic Biology and Chemistry courses quickly, all the better - but make sure you’re rock-solid on the basic courses, even if you studied these basics when you were 14. You can’t afford to be fuzzy on those. Then progress through the AP levels in biology, chemistry, and calculus.
I don’t think the greatest difficulty for you would be academics, but rather the lifestyle. UM has a lot of wealthy students, lots of conspicuous consumption, a lot of partying, lots of “good looking/tan” teenagers who value looks (Miami!) or “work hard/play hard”, which in the US means to study to get good grades during the week, then party a lot and get very drunk on weekends.
It can be difficult unless you’re already wealthy yourself and/or have a good “beach bod”.
Obviously UM will have lots of strong students, lots of premeds, lots of students who want to succeed, lots of difficult classes where you need to study hard and smart.
But based on everything I know, the % of “good looking/wealthy” kids who are into “work hard/play hard” is higher than what the USNWR ranking would let you think.